Alkalinity | EPFL Graph Search

Alkalinity (from al-qaly) is the capacity of water to resist acidification. It should not be confused with basicity, which is an absolute measurement on the pH scale. Alkalinity is the strength of a buffer solution composed of weak acids and their conjugate bases. It is measured by titrating the solution with an acid such as HCl until its pH changes abruptly, or it reaches a known endpoint where that happens. Alkalinity is expressed in units of concentration, such as meq/L (milliequivalents per liter), μeq/kg (microequivalents per kilogram), or mg/L CaCO3 (milligrams per liter of calcium carbonate). Each of these measurements corresponds to an amount of acid added as a titrant. In freshwater, particularly those on non-limestone terrains, alkalinities are low and involve a lot of ions. In the ocean, on the other hand, alkalinity is completely dominated by carbonate and bicarbonate plus a small contribution from borate. Although alkalinity is primarily a term u

Statistical analysis of the spatial and temporal variability of polarimetric weather radar signals in the Alps

Floortje Elisabeth Maria Van den Heuvel

The accurate estimation of precipitation quantities arriving at the ground in complex terrain such as the Alps is complicated by many factors. The orography interacts with atmospheric flow and thus influences the spatial and temporal distribution as well as the microphysics of precipitation. Although weather radar can provide high-resolution measurements of precipitation, their view is sometimes blocked by the relief in which case measurements from higher altitudes need to be extrapolated to the ground level. When extrapolating the radar measurements aloft for quantitative precipitation estimation (QPE) at the ground, these must first be corrected for the vertical change of the radar echo caused by the growth and transformation of precipitation (VPR correction). Many existing operational algorithms for QPE and VPR correction assume the vertical structure of precipitation to be spatially and temporally homogeneous. However, given the variable nature of precipitation this assumption may not hold, especially in mountainous areas. This thesis seeks to contribute to the improvement of QPE by radar in the Alps through the analysis of the spatio-temporal variability of the polarimetric radar signals. More specifically, this is done through contributions to questions on radar monitoring and stability, the characterisation of the spatio-temporal variability of the melting layer and the study of the potential for the inclusion of polarimetric radar variables in a more localised vertical profile correction approach. A method based on spectral analysis is used to provide some new perspectives on radar hardware monitoring using the polarimetric signals returned by a single bright scatterer. It is shown that valuable information on the state and stability of the radar hardware can be obtained if different scales of variability and several polarimetric variables are considered. The same approach is used for the characterisation and comparison of the spatio-temporal variability of the melting layer on the relatively flat Swiss plateau and in a large inner Alpine valley in the Swiss Alps. Based on the results of this study it appears that the smaller spatial scales contribute more to the total spatial variability of the melting layer in the case of the Alpine environment. Finally, building on the availability of polarimetric data and a hydrometeor classification algorithm, a new framework for the application of machine learning methods to study the vertical structure of precipitation in Switzerland as well as a more localised vertical profile correction method is proposed. It is shown that models which include information on hydrometeor proportions better represent the observed patterns of vertical change in precipitation and that these models can predict from altitudes between 500 to 1000 metres higher than models based on only reflectivity data.

EPFL2019

Urban Greenhouse Gas Emissions Monitoring in Davos, Switzerland, Before, During and After the World Economic Forum Annual Meeting 2012

Michael Lehning

Efforts to reduce anthropogenic greenhouse gas emissions require validation. Atmospheric measurements capture all emissions, and provide a unique and powerful means of continuous validation and feedback. To demonstrate the utility of real time greenhouse gas measurements, in-situ GHG mixing ratio instruments were deployed in Davos, Switzerland to measure emissions from the city before, during and after theWorld Economic Forum (WEF). Three Instruments were deployed at two separate locations over 3 months (late December 2011 to February 2012). One site was located in the middle of the Davos urban area and a second site was located out of the valley in the surrounding mountains. Carbon Dioxide (CO2), Methane (CH4), Carbon Monoxide (CO) and water vapor (H2O) were measured continuously by Picarro G2401 instruments at both sites. Additionally, a Picarro flux analyzer was deployed in the city to evaluate the inverse fluxes. The mesoscale atmospheric model, WRF nudged to meteorological observations (WRF-FDDA), was used to simulate the transport of GHG over the valley of Davos at 1.3km resolution. A Mini Micro Pulse LiDAR (MiniMPL) from Sigma Space was deployed to evaluate the simulated planetary boundary layer depth from the WRF-FDDA model. The initial flux estimates for CO2 were constructed based on inventories reported for 2005. CO2 mixing ratio measurements prior to WEF suggest the difference between modeled (real-time) and inventory (annual) emissions to be on the order of +40%. The enhancement is likely due to the increased use of heating fuel in the winter.We present here the temporal variability in the inverse fluxes, which are correlated with a cold wave severely affectingWestern Europe during the past winter, as well as changes in anthropogenic activities during the week of the WEF meeting. Also presented are new analyses of composite diurnal cycles of hourly CO/CO2 ratios, which provide additional information on the contributions of traffic relative to heating fuel. The absence of traffic peaks during the WEF meeting, indicate a change in road emissions potentially responsible for the observed decrease in the city emissions during the meeting. Acknowledgments: Calibration tanks were provided by C. Sweeney, NOAA ESRL.

2013

Radar processing techniques to study summer snowfall in the Sør Rondane Mountains, Antarctica

Alfonso Ferrone

Snowfall is one of the main positive contributors to the surface mass balance of the Antarctic ice sheet. There are several techniques to monitor solid precipitation on the continent. Among them, meteorological radars provide the means to characterize the phenomenon on a local scale, with an unparalleled view of the processes in the lower levels of the atmosphere. However, due to the harsh weather and the logistic difficulties associated with their deployment in Antarctica, their usage is often limited to the immediate surroundings of the few scientific bases scattered on the continent.In this thesis, meteorological radar observations are used to investigate the effect of complex orography on precipitation in the vicinity of the Belgian research base Princess Elisabeth Antarctica (PEA). Two algorithms for the processing and calibration of the radar measurements collected had to be developed and evaluated. One of them has been applied to the data from three K-band Doppler profilers (MRR-PRO), deployed in a transect across the Sør Rondane Mountains, south of PEA. These datasets have been used together with atmospheric model simulations, reanalysis, and complementary observations to examine the role of sublimation and other local phenomena on precipitation at the three sites. The possible connections with the synoptic conditions on a larger scale have also been investigated.The first part of the thesis is devoted to the development of two algorithms. First, we introduce a differential reflectivity calibration method for dual-polarization weather radars with vertical scanning capability. Even though the technique functions with solid and liquid precipitation, it is particularly suited for radars deployed in polar regions with no access to rainfall. The offset derived from each vertical scan undergoes an Ordinary Kriging interpolation, which determines the temporal variability of the calibration. A processing algorithm for the MRR-PRO measurements has also been developed. The method improves the sensitivity of the radar products and the consistency of the Doppler velocity field. At the same time, it addresses some artifacts in the data caused by interference and noise within the instrument.The second part of the thesis revolves around the analysis of the datasets collected at PEA. The MRR-PRO measurements are used to investigate the presence of virga and surface precipitation at the three sites. Moreover, a series of high-resolution simulations reveal similarities in the wind speed and relative humidity profiles in the lower troposphere. The varying depth of a dry layer in the lowest levels causes differences in sublimation across the transect. Additionally, analysis of the wind direction indicates that orographic lifting enhances precipitation near the mountain peaks. Finally, our observations, consistent with previous studies, suggest that virga and surface precipitation represent different stages of the same large-scale precipitation systems.Altogether, this thesis proposes a series of tools for studying snowfall through radar observations. Moreover, the analysis of the interaction between orography and precipitation at PEA covers locations that have not yet been the subject of scientific studies. It demonstrates the existence of a significant degree of variability on relatively small scales, highlighting the effect of complex terrain and its interaction with the local and large-scale circulation on snowfall in the region.

EPFL2022